Add visual editor component for configuring a model-check test

[chrispcampbell]

Currently the only way to develop a new model-check test is to edit YAML code, but the syntax is not easy to work with or to remember. (See Testing and Comparing Your Model wiki page for some guidance.)

It would be better if we had a visual editor inside the model-check app that allows the user to configure a new test or edit an existing one. It should have a preview graph on the right side that updates in real time when the test parameters are changed, and it should allow for viewing and copying the resulting YAML code.